Double-rotor shears are known in which the cutting edges of the blades are practically parallel to the two axes of rotation. In this case cutting is effected substantially simultaneously throughout the width of the strip. The instant forces to be supplied by the machine are then large and sudden, cause a very disturbing noise in operation and cause rapid blade wear.
Double rotor shears are also known in which the cutting edge of one of the blades is parallel to the axis of rotation of this blade, while the cutting edge of the other blade is inclined with respect to the corresponding axis of rotation and does not lie in the plane passing through said axis. The two cutting edges thus form an angle with each other, called the cutting angle, such that cutting becomes progressive because the point at which the two cutting edges intersect at any one point in time, or cutting point, moves along these edges as cutting progresses. The instant forces and the noise are considerably reduced. But the cutting point does not remain in the plane of travel of the strip to be cut, and the inclined cutting edge cannot be rectilinear. This cutting edge thus cannot be physically brought about by the intersection of two planes. As a result, not only fabrication of the blade involves delicate and costly machining, but in addition each resharpening of this blade is difficult since a curved surface has to be regenerated.